A gas turbine generally includes an inlet section, a compressor section, a combustion section, a turbine section and an exhaust section. The inlet section cleans and conditions a working fluid (e.g., air) and supplies the working fluid to the compressor section. The compressor section progressively increases the pressure of the working fluid and supplies a compressed working fluid to the combustion section. The compressed working fluid and a fuel are mixed within the combustion section and burned in a combustion chamber to generate combustion gases having a high temperature and pressure. The combustion gases are routed along through a hot gas path into the turbine section where they expand to produce work. For example, expansion of the combustion gases in the turbine section may rotate a shaft connected to a generator to produce electricity.
The combustion section generally includes a plurality of combustors annularly arranged about an outer casing. In lean premix style combustion systems, each combustor includes one or more premix type fuel nozzles. A typical premix fuel nozzle includes a center body that is at least partially surrounded by an outer tube or sleeve. A premix flow passage is defined between the outer sleeve and the center body. Multiple vanes or struts extend between the center body and the outer sleeve within the premix flow passage.
In operation, fuel is injected into compressed air flowing through the premix flow passage. The vanes impart angular swirl to the compressed air thus enhancing mixing with the fuel upstream from a combustion zone of the combustor. The premixed fuel-air is generally a fuel-lean mixture. The fuel-lean mixture burns more efficiently, thus reducing CO emissions and producing lower NOx emissions than diffusion flame technology.
At least one of the premix type fuel nozzles may include a pilot nozzle. The pilot nozzle may be coaxially aligned with and disposed within the center body of the corresponding fuel nozzle upstream from the combustion zone. During particular combustion operation modes, the pilot nozzle may deliver a premixed fuel and air mixture to the combustion zone to produce a premixed pilot flame. The premixed pilot flame is generally used to ensure flame stability as the combustor is operated in certain modes and/or when the combustor transitions between various modes of operation.
In order for the pilot nozzle to function, pilot or compressed air and pilot fuel must be supplied through the center body to the pilot nozzle. However, space restrictions within the center body may limit possibilities for routing the pilot air and fuel to the pilot nozzle, thus potentially limiting overall effectiveness of the pilot nozzle. Therefore, an improved fuel nozzle assembly having a pilot nozzle would be useful in the technology.